Project Summary Severe psychiatric disorders, including schizophrenia, bipolar disorder, and major depressive disorder are prevalent, chronic and debilitating brain diseases that result in intense suffering and reduced quality of life. There is a profound lack of new medicines for psychiatric disorders, which remain a major area of unmet medical need. Discovery of more effective therapies for mental illness has been limited by poor understanding of disease biology, a lack of model systems that can predict clinical results, and the dearth of tools to rapidly study relevant electrical, synaptic, and circuit-level phenotypes that underlie many psychiatric disorders. In this proposal, we aim to address these challenges. One powerful platform for studying brain function in intact neural circuits is rodent brain slice: cells buried deep in the brain can be studied, but maintain many of their native connections and properties. However, it has been challenging to comprehensively analyze neuronal properties and connectivity in brain slices, since traditional methodology based on patch clamp, the piercing of cells with microelectrodes, is extremely labor-intensive and lacks sufficient neuronal throughput. A promising solution to this problem is optogenetics, wherein which light is used to stimulate and record neuronal activity. The Optopatch platform recently developed at Q-State Biosciences, comprised of engineered optogenetic proteins, custom microscopes, and software, enables simultaneous stimulation (blue light) and recording (red light) of electrical activity from ~100 neurons in a dish with one millisecond temporal resolution, single cell spatial resolution and high signal-to-noise ratio. Patterned blue light can be used to stimulate one or more neurons while recording from all synaptic partners. Hence, the Optopatch platform can be used to study single-cell excitability, synaptic transmission, and network behavior. Here we propose to create a custom microscope for harnessing the Optopatch system in brain slices and demonstrate applicability of the system to compound screening. Since background fluorescence and light scattering are many times higher in tissue than in dishes of cultured neurons, a new microscope design is required for high-speed voltage imaging. We will employ reconfigurable, patterned illumination to selectively excite a small subset of cells, reducing the background to manageable levels by minimizing the total amount of light delivered. We will also drive high expression of Optopatch proteins in a small fraction of neurons to obtain large signals with minimal cross-talk from neighboring cells. Finally, we will confirm that the system can be used to efficiently determine firing behavior of different types of neurons and measure pharmacological responses. Ultimately, the system we propose to develop has potential to benefit patients with mental illness by enabling new, circuit-based therapeutic discovery approaches.